It is a direct transliteration. In dialect Arabic it is often used, but it will never be used if you want to write a letter, of speak politely. It is mostly used with friends or children. Family members or neighbours, teachers or buyer-seller will not use this. It is too foreign to be used amongst family members, and too personal for respectful relations. More, since it is not Arabic, there are religious and Arabic loving people who take an offence in mixing it in an Arabic sentence. I advise only using it if you have a close Arabic friend who is open minded.
Funfact: This formula is even used on a daily basis in Israel, and even not only between Arabic speaking people (!), so there's even a joke about the modern Hebrew language in all other languages, than English that sound less funny in English, but you can imagine: - How do you say goodbye in Hebrew? - Yalla, bye! :)
(and what is funny about this, if I have to explain: it's Arabic and English, no Hebrew in it. Btw it's quite slang or disrespectful, but actually there are much less situations in the Hebrew language culture today, when you should not use it, I mean I can totally imagine telling this even to Israel's president or prime minister, who is called by the whole country just Bibi e.g., just like a friend from the kindergarten... :D )